As mentioned, Passport America and Good Sam memberships give you discounts in many campgrounds/RV parks (except in the Southern US in the winter). The snowbirds have the money sucked out of them in Florida and other warm weather areas in the winter. No discounts and inflated rates so you may want to shop around and do it now rather than wait until it gets cold again as campsites can be hard to find then. If you don't mind a little freezing weather but no snow you can get better rates a little further North. State parks and COE (Corps of Engineers) campgrounds usually don't raise their rates in winter but they will limit your stay so keep that in mind. Then there is Quartzite, AZ where you have the whole desert to camp in and BLM (Bureau of Land Management) areas in the SW US that have lots of boondocking available. Also check http://rvparkreviews.com .
You may also want to check http://escapees.com